You know, I guess we're living in pretty good times. In
my Dads generation, if you wanted to see a beautiful red head, you had to watch
"I Love Lucy" and in black and white at that! But these days we
have color TV and the beautiful red heads no longer play silly housewives, they
play FBI investigators on the X-Files. Gillian Anderson was kind enough to
consent to an interview, but our only problem was which RouterGod staffer would
get the privilege? The entire "Gang of Nine" wanted to interview
Gillian, but in the end the honor fell to Michael McClenney because he hasn't
got to meet a celebrity since Tonya Harding was here. Anyway, this is just
an overview on switching, we will get into more details in upcoming
installments. Gillian was born in Chicago, Illinois August 9, 1968.
At one time she actually married a man named Clyde Klotz but could not bear
being referred to as Gillian Klotz so she soon divorced. She moved out to
California because she wanted to work as an actress in Canada and did so for the
first 5 years of X-Files. Now she is one of the few American actors lucky
enough to work in the United States. Without further adieu, Gillian
Anderson:

Aren't those beautiful earrings?

I see this guy on Gillian's show,
sometimes he'll do something that distracts
me as I'm watching Gillian. I think he's
a stagehand or something...

Hi ya cutie, you can call me Gillian. And I'm happy to be here,
I hope I can answer your switching questions.

Michael

Let's get started then. Here at RouterGod Online Magazine we're
only interested in Cisco switches and routers, is that a problem?

Gillian

Certainly not! You know, when I was a struggling actress, I had
to put up with a lot of cheap crap. I lived in a crummy apartment
and had this horrible HP ProCurve switch. The really nice thing
about becoming successful is you are able to afford nice things like
houses, cars and Cisco switches. I'll never forget the day that I
was hired to play Agent Scully on the X-Files, the first thing I did was
go out and buy a brand new Catalyst 2924 XL. What a wonderful,
wonderful switch...

Michael

Very good, switches have become very popular, can you tell us why?

Gillian

Sure cutie, for one thing the cost of switches has come way
down. They can directly replace hubs too. You can literally
replace a hub with a switch in seconds, no configuration required.

Michael

Whoa, whoa, wait a minute. Switching is a huge subject, there
are issues about spanning tree and trunking protocols and VTP domains
and etherchannel...what about that? What about that?

Gillian

Oh boy! You just got worked up there, didn't you? Relax
cutie, you are talking advanced CCNP type concepts there, I'm just a
CCNA but let me explain what I mean. Cisco's philosophy about
switches is "do no harm". All ports are by default in
the same vlan, vlan1 to be exact. So you can plug in the switch
and plug in the cables and now every port is in a separate collision
domain.

Michael

Many of our viewers are still fixated on your picture at the top of
the page, can you explain about collisions domains, for their benefit.

Gillian

Not a problem Michael, I get that all the time. Switches work
at layer 2 of the OSI model. They listen on each port and write
down the source MAC address of every Ethernet frame they hear.
This is put in what Cisco calls a CAM table, it's a listing of every
port and what machines can be found on each port. These ports are
not connected to one another. The switch can connect them when it
wants to but otherwise they are not connected.

Michael

Nobody ever put it like that, so the switch is just a series of ports
that are not normally connected, under what circumstances does the
switch connect all the ports?

Gillian

Connect all the ports? You would have to be more specific, is
there any multicasting going on? Are there different vlans?

Michael

No, you know what I mean, why would the switch decide to connect 2
ports together?

Gillian

Now that's a different question, you said "all ports" at
first. Let me start from the beginning. We're only talking
one vlan right now. Remember the switch pretty much knows what
machines are on what ports, it's been listening and creating these
entries in the CAM table. So it receives a frame coming in on port
number 1. It looks at the frame and reads the MAC address of the
destination and checks it's CAM table to see if it has already heard
that machine on another port, if it has, it will forward that frame out
the port that the destination machine resides on. No other ports
will hear this transmission. On a single switch in a single vlan
you could have a machine on port 3 sending a file to a machine on port
21, a machine on port 17 sending email to a machine on port 9, a machine
on port 11 logging in to a server located on port 16, all at the same
time. If you tried to do that with a hub, you would have
collisions as only one data stream could travel the Ethernet at one
time.

Michael

Excellent. In your example the switch has tripled the available
bandwidth. What if the switch looks in the CAM table but there is
no entry listing the MAC address of the destination machine? Does
the switch just drop the packet?

Gillian

Listen up, cutie! You just said "packet"! A
packet is a layer 3 data unit. We use frames thank you very
much. And no, the switch will not drop the frame. If the
switch does not have an entry in it's CAM table, it will copy the frame
and send it out all ports at once. When it gets a response it will
add that machine to the CAM table and all future frames will be
switched to that port.

Michael

OK, now I know. What about broadcasts, what does the switch do
with them? You know, ARP broadcast and stuff like that?

Gillian

Broadcasts are always flooded out all ports that are members of the
same vlan.

Michael

There! you said it again, vlan! What's a vlan?

Gillian

My goodness! I love your enthusiasm! A vlan or
"virtual lan" is simply a group of ports that you have put in
the same broadcast domain. Take a 24 port switch for example, by
default all ports can hear broadcasts from all other ports. The
switch is maintaining a CAM table of all 24 ports so that it can switch
frames between ports. This is a single lan, or vlan if you like,
now take a hacksaw and saw the switch in half. Now you have 2
vlans. Computers in one vlan cannot talk or hear broadcasts from
the other vlan.

Michael

So instead of actually sawing a switch into pieces, you can
administratively create different vlans and assign ports to these vlans?
Am I getting that right?

Gillian

Exactly, the only switches that come with hacksaws are SMCs and the
lower priced Addtrons. On a Catalyst you create these separate vlans
from the command line. Each
port can only be a member of one vlan though, there are exceptions like
SPANning a port and trunking but for now, the rule is one vlan per port.

Michael

Ok, here's a stupid question, how does this really work in the real
world? How do subnets figure in? This is kind of confusing.

Gillian

You are such a cutie! You've got it figured out and you don't
even know it! Every vlan is a separate subnet! You create a
vlan called Accounting, all the computers in that vlan have IP addresses
that place them in the same subnet. All machine on the same subnet
(vlan) hear broadcasts from the other machines on the same vlan.

Michael

How does the switch route traffic from one vlan to another?

Gillian

They don't.

Michael

If the switches don't route the packets from one subnet to another,
how do the packets get routed?